The secure and timely disposal of transuranic materials (primarily plutonium) has been the subject of intense debate in recent years. The two primary sources of transuranics are disassembled nuclear weapons and spent fuel from existing power reactors.
Large quantities of weapons-grade plutonium currently exist in U.S. and C.I.S. weapons stockpiles which are to be dismantled in accordance with current arms reduction treaties. This material must be closely safeguarded to prevent its diversion for use in nuclear weapons. The weapons-grade material can be quickly put into a form which is not readily usable for nuclear weapons by diluting the material and introducing radioactivity, thus rendering it difficult to handle. However, this short-term "disposal" of weapons material does not remove the long-term proliferation risk, and such denatured material requires perpetual active safeguarding. Therefore, the only enduring solution is to actively safeguard the denatured weapons material in the short-term and eventually destroy the material.
Even larger quantities of transuranics are contained in the spent fuel inventories of existing nuclear reactors. This material does not pose an immediate proliferation concern because it already exists in a dilute (transuranics constitute about 1% of the total heavy metal mass) and radioactive form. The current U.S. waste management strategy calls for direct disposal of the LWR spent fuel in a centralized repository. However, direct disposal of LWR spent fuel in a centralized repository is complicated by the presence of the transuranics, which dominate the long-term radiotoxicity of LWR spent fuel. Thus, from a waste management perspective it is desirable to process the LWR spent fuel to remove the transuranics and process the remaining waste material into a more stable form. Subsequent destruction of the separated transuranic material reduces its long-term radiological and proliferation hazards.
Therefore, sufficient motivation exists for eventual destruction of the transuranics from both disassembled weapons and spent fuel: i.e., the reduction of proliferation and radiotoxicity hazards. Destruction by fission is the only means available to permanently destroy the transuranics. Although fission creates radioactive fission products which have a higher short-term hazard than the original fuel material, the fission products decay much more rapidly, so the long-term hazard is significantly reduced. Furthermore, the energy produced by the fission reactions can be converted to electrical power (the fission of 1MT of actinides yields enough energy to produce approximately 1GWe-year of electricity), and the sale of this power allows revenue recovery for the disposition activity.
In all conventional fission nuclear reactor systems, the transuranic destruction rate is mitigated by in-situ production of Pu-239 (by U-238 neutron capture). The available range of destruction/production characteristics in metal-fueled cores allows a flexible transuranic management strategy. Conventional fast reactor cores maintain or even increase the transuranic inventory (conversion ratio of 1.0-1.3); this allows sustained power production from a fixed transuranic inventory. By removing fertile material and/or altering the neutron balance, the conversion ratio can be reduced. Core designs with conversion ratios between 0.5 and 1.0 have been investigated; further reductions in the conversion ratio would require transuranic contents greater than 30 weight percent, as previously investigated in the Integral Fast Reactor (IFR) metal fuels testing program. The partial burner core designs, with 0.5-1.0 conversion ratios, are referred to as conventional burner designs because they utilize conventional IFR metallic fuel alloys.
Because the minimal conversion ratio of conventional burners is 0.5, they can achieve transuranic consumption rates of roughly half the maximum value (1/2.times.1 g/MW.sub.t d). To allow more rapid destruction of the transuranics, non-conventional metal fuel alloys are required; to achieve the maximum transuranic consumption rate of 1.0 g/MW.sub.t d, a non-uranium fuel form is required. Preliminary neutronic investigations of non-uranium core designs (called pure burners because they achieve the maximum destruction rate) have been discussed in R. N. Hill, D. C. Wade, E. K. Fujita, and H. Khalil, "Physics Studies of Higher Actinide Consumption in an LMR." International Conference on the Physics of Reactors Marseille, France, Apr. 23-27, 1990, p.1-83; R. N. Hill, "An Evaluation of Reactivity Coefficients for Transuranic Burning Fast Reactor Designs," Transactions of the American Nuclear Society, Vol. 65, p. 450 (1992); and GE Nuclear Energy, "Plutonium Disposition Study," GEFR-00919, May 1993. However, the fuel material design properties and behavior were not investigated in any detail.